Cholestatic liver disease has many causes, including autoimmune destruction of bile duct epithelial cells, genetic disorders, and physical obstruction of the bile duct. Currently, the only approved treatment for many types of cholestatic liver disease is usodeoxycholic acid, which is only efficacious in a small fraction of patients. Although, several studies indicate an important role for inflammation in the development of hepatocellular injury and fibrosis during cholestasis, glucocorticoids and other potent anti-inflammatory drugs that target classical inflammatory cytokines are only modestly effective at ameliorating liver disease in patients and are often associated with severe side-effects. The failure of current anti-inflammatory treatment regimens to treat cholestatic liver disease underscores the importance of elucidating the mechanism(s) involved in the initiation of hepatic inflammation in this type of liver disease. Our preliminary studies demonstrate that pathological concentrations of the bile acid, taurocholic acid (TCA), stimulate hepatocytes to produce the cytokines, IL-23 and macrophage inflammatory protein-2 (MIP-2). Activation of Akt by TCA is required for upregulation of IL-23 and MIP-2, whereas activation of p38 by TCA inhibits upregulation of these cytokines. IL-23 contributes to hepatic inflammation by stimulating Th17 cells to produce IL-17A. This cytokine inhibits TCA-mediated activation of p38 in hepatocytes leading to enhanced production of MIP-2 and IL-23. Our studies demonstrate further that activation of lysophosphatidic acid receptor 1 (LPAR1) is required for upregulation of IL-23 and MIP-2 in TCA-treated hepatocytes, and that LPAR1 is activated independent of its ligand, LPA. This indicates that LPAR1 is activated by a novel mechanism that may involve direct activation of LPAR1 by TCA. Based upon these studies, the central hypothesis of this application is that during cholestasis, elevated concentrations of bile acids activate LPAR1 which increases expression of IL-23 and MIP-2, a process which is enhanced by IL-17A-mediated inhibition of p38. The studies in this proposal aim to test the central hypothesis by: (1) determining the mechanism by bile acids increase expression of IL-23 and MIP-2 in hepatocytes, (2) determining the mechanism by which IL-17A inhibits activation of p38 by TCA, and (3) determine the mechanism by which p38 negatively regulates IL-23 and MIP-2 induction by TCA. Collectively, these studies will increase our current knowledge of regulation of hepatic inflammation during cholestasis, which will allow for the identification of new drug targets to tret cholestatic liver disease.